1. Field of the Invention
The present invention relates to a system for actuating intake and exhaust valves in stepped movement with a linear stepping motor disposed on a stem end of the intake and exhaust valves of an engine.
2. Description of the Related Art
Some conventional actuating systems for opening and closing intake and exhaust valves includes a single camshaft which has cams for the intake and exhaust valves, the camshaft being disposed above or laterally by an engine. The camshaft is connected to the crankshaft of the engine by a belt or the like, so that the camshaft can rotate synchronously with the rotation of the engine. The valves have stems whose ends are pressed by cam surfaces of the camshaft through a link mechanism such as rocker arms or push rods. The intake and exhaust valves are normally closed by springs, and can be opened when their stem ends are pressed by the cam surfaces.
In other valve actuating systems, an intake camshaft having cams for acting on intake valves and an exhaust camshaft having cams for acting on exhaust valves are disposed above an engine. The intake and exhaust valves are opened when the stem ends of the intake valves are directly pushed by the cam surfaces of the intake camshaft and the stem ends of the exhaust valves are directly pushed by the cam surfaces of the exhaust camshaft.
However, the above conventional actuating systems for opening and closing intake and exhaust valves have several disadvantages. First, the conventional systems include camshafts and link mechanisms added to the engine. This necessarily renders the engine large in size.
Secondly, since the camshafts and the link mechanisms are driven by the output shaft of the engine, the engine output power is partly consumed by the frictional resistance produced when the camshafts and the link mechanisms are driven by the engine. As a result, the effective engine output power is reduced.
Finally, the timing with which the intake and exhaust valves are opened and closed cannot be altered during operation of the engine, but the valve opening and closing timing is preset such that the engine operates with high efficiency only when it rotates at a predetermined speed. Therefore, the engine output power and efficiency are lower when the engine rotates at a speed different from the predetermined speed.
To solve the above problems, there have been proposed valve actuating systems for opening and closing intake and exhaust valves under electromagnetic forces from electromagnets, rather than with camshafts, as disclosed in Japanese Laid-Open Patent Publications Nos. 58-183805 and 61-76713.
However, with the electromagnets disclosed in the above two publications, the distance between the electromagnet and the magnetic body varies when the intake and exhaust valves are actuated, resulting in an abrupt change in electromagnetic forces and highly unstable operation of the intake and exhaust valves.